Liquid crystal display devices, being characterized by their thinness and light weight, have successfully found commercial applications as color display devices. Among these color liquid crystal display devices, transmissive liquid crystal display devices provided with a light source for illumination from behind are in particularly widespread use, and are adopted for an increasingly wider variety of applications because of the above-mentioned features.
In contrast to the transmission liquid crystal display device, the reflective liquid crystal display device does not require a backlight for display, and therefore can reduce the power consumption of the light source. The exclusion of the backlight further characterizes the reflective liquid crystal display device by allowing it to be more compact and lightweight.
In other words, in comparison to conventional transmissive liquid crystal display, the reflective liquid crystal display device can lower the power consumption, and be suitably used in equipment which needs to be lightweight and thin. For example, if the equipment with the reflective liquid crystal display device is designed while retaining conventional operation time, the reflective liquid crystal display device can not only cut down on the backlight space and weight, but consumes less power, and becomes capable of running on a smaller battery, making it possible to further reduce the size and weight. If the equipment with the reflective liquid crystal display device is manufactured while retaining conventional size or weight, use of a larger battery is expected to increase operation time dramatically.
In addition, as to display contrast properties, the light emitting display device, such as the CRT, degrades greatly in contrast ratio when used outdoors during the daytime. Even the transmissive liquid crystal display device subjected to a low reflection treatment inevitably suffers similarly from greatly decreased contrast ratios when used in ambient light, such as direct sunlight, that is excessively strong compared to display light.
In contrast, with the reflective liquid crystal display device, the display light obtained is proportional to the amount of ambient light, which is an especially suitable feature for application in a personal digital tool, a digital still camera, a portable camcorder, and other devices that are often used outdoors.
When considering these potential application fields, the reflective color liquid crystal display device appears very promising; however, a relatively low contrast ratio and reflectance, as well as insufficient performances in multi-color, high precision, and moving picture display, have so far been obstacles in realizing commercially viable reflective color liquid crystal display device.
The following description will explain the reflective liquid crystal display device in further detail. The conventional twisted nematic (TN) type liquid crystal element includes two linear polarizer plates (hereinafter, will be simply referred to as polarizer plates), and therefore boasts an excellent contrast ratio and viewing angle dependency property; however, the reflectance is inevitably low. In addition, since the liquid crystal modulation layer is separated from the light reflective layer by a distance equivalent to the thickness of a substrate, etc., there occurs parallax due to a disparity between incoming and outgoing optical paths of illumination light. Therefore, especially in a typical arrangement used for transmissive liquid crystal display devices where a single liquid crystal modulation layer is combined with a color filter that includes a separate subpixel for each color element, provided that light does not travel parallel to the normal to the substrate, ambient light enters and exits after reflection through different color subpixels. This causes moire and other undesirable phenomena, rendering the transmissive liquid crystal display device unsuited for high resolution, high precision, color display use.
For these reasons, no reflective color display device using this display mode has so far been commercialized.
Meanwhile, Guest-Host type liquid crystal elements (hereinafter, will be abbreviated as GH) have been developed that uses no or only one polarizer plate and includes liquid crystalline material doped with dyestuff. However, the GH type liquid crystal element is not highly reliable due to the addition of the dye, and the low dichroic ratio of the dye cannot produce a high contrast ratio.
Among these problems, the insufficient contrast level in particular causes serious degradation in color purity and creates a need to incorporate a color filter of high color purity in a color display device using a color filter. This entails a problem of reduced brightness caused by the color filter of high color purity, and cancels to some degree the advantage of this mode that high brightness is achieved by use of no polarizer plates.
On these backgrounds, research and development is under way to successfully manufacture a liquid crystal display element in a mode in which a single polarizer plate is used (hereinafter, will be referred to as a single polarizer plate mode), which is highly promising to realize a high resolution and high contrast display.
Japanese Laid-Open Patent Application No. 55-48733/1980 (Tokukaisho 55-48733) discloses such an example of a liquid crystal display element of a reflective TN mode (45°-twisted type) using a single polarizer plate and a quarter-wave plate.
With this liquid crystal display device, black and white display is performed, using a 45°-twisted liquid crystal layer and controlling the electric field applied thereacross, by realizing two states, in one of which the plane of polarization of incoming linearly polarized incident light is parallel to the optical axis of the quarter-wave plate and in the other of which the plane of polarization forms 45° with the optical axis of the quarter wave plate. The liquid crystal cell is structured to include a polarizer, a 45°-twisted liquid crystal cell, a quarter-wave plate, and a reflector plate, when viewed from the side at which light enters.
Further, U.S. Pat. No. 4,701,028 (Clerc et al.) discloses a liquid crystal display device of a reflective-type, homeotropic alignment mode wherein a combination of a single polarizer plate, a quarter-wave plate, and a perpendicularly aligned liquid crystal cell is used.
Meanwhile, the inventors of the present application filed an application for a reflective-type, parallel alignment mode wherein a combination of a single polarizer plate, a homogeneous alignment liquid crystal cell, and an optical retardation compensation plate is used (see Japanese Laid-Open Patent Application No. 6-167708/1994 (Tokukaihei 6-167708)).
This reflective liquid crystal display device includes a liquid crystal cell constituted by a homogeneously-aligned liquid crystal layer, a reflector plate (disposed inside the liquid crystal cell beneath the liquid crystal layer), a polarizer plate (disposed on the liquid crystal cell), and a single optical retardation compensator plate (placed between the liquid crystal cell and the polarizer plate). Further, according to this display mode, throughout the total length of the optical path, i.e., the incoming optical path and the outgoing optical path, light passes through the polarizer plate only twice and through the transparent electrode where light is inevitably absorbed on a glass substrate (top substrate) of the liquid crystal cell also only twice. Therefore, a high reflectance can be obtained by means of a reflective liquid crystal display device of this structure.
Further, Japanese Laid-Open Patent Application No. 2-236523/1990 (Tokukaihei 2-236523) discloses an arrangement in which a twisted nematic liquid crystal layer is interposed between a reflector plate (disposed inside a liquid crystal cell) and a single polarizer plate.
Further, Fourth Asian Symposium on Information Display (Chung-Kuang Wei et al., Proceedings of The Fourth Asian Symposium on Information Display, 1997, page 25; hereinafter will be abbreviated as ASID 97) discloses an arrangement wherein 90°-twisted nematic liquid crystal is interposed between a reflector plate disposed inside the cell and a combination of a quarter-wave plate and a polarizer plate which realizes a broad band display.
In addition, Japanese Laid-Open Patent Application No. 4-116515/1992 (Tokukaihei 4-116515) discloses a liquid crystal display device wherein incident circularly polarized light is used for display. In addition, as a method of obtaining circularly polarized light in a broad band of spectrum, Pancharatnam teaches the use of a plurality of optical retardation compensator plates in Proc. Ind. Acad. Sci. Vol. XLI, No. 4, Sec.A, page 130, 1955.
The description below will explain display principles of a single polarizer plate mode employed in ASID 97 and in the Japanese Laid-Open Patent Applications No. 6-167708/1994, No. 2-236523/1990, and 4-116515/1992.
The polarizer plate disposed on the side where light enters serves to pass only one of the linearly polarized light components of the incoming and outgoing polarized light and absorb the other linearly polarized light component. The polarization state of the incoming light that has passed through the polarizer plate is then changed by an optical retardation compensator plate, such as a quarter-wave plate (in the cases of Japanese Laid-Open Patent Application No. 6-167708/1994 and ASID 97), or remains unchanged (in the case of Japanese Laid-Open Patent Application No. 2-236523/1990), and the light enters the liquid crystal layer. The polarization state is changed further as the light passes through the liquid crystal layer, before the light reaches a reflector plate.
Further, the light that has reached the reflector plate changes its polarization state in the reverse sequence to that of the incoming light: the light passes through the liquid crystal layer, the quarter-wave plate, etc. Consequently, the ratio of the linearly polarized light component in a transmission direction of the polarizer plate to the light obtained here will decide the total reflectance of the liquid crystal layer. In other words, the liquid crystal display element appears brightest when the outgoing light, immediately before passing through the polarizer plate, is linearly polarized in the transmission direction of the polarizer plate, and darkest when linearly polarized in the absorptive direction of the polarizer plate.
It is known that the necessary and sufficient condition for the light which enters and leaves the liquid crystal display device perpendicularly to the device to realize such a bright state is that the light be linearly polarized in an arbitrary direction on the reflector plate, and that for it to realize such a dark state is that the light be circularly polarized either right handed or left handed on the reflector plate.
Meanwhile, a touch panel, as well as a conventional keyboard, is a very useful input means incorporated in a personal digital tool. This is especially true in inputting such languages including Japanese that keyboard inputs need to be converted; with increasing information processing capability and newly developed software, the touch panel, which used to serve simply as a pointing device, now more typically plays a greater role as an input device such as a pen-based handwriting input device.
To realize this particular input method, the input device is disposed to overlap the front of the display device. However, since the reflective liquid crystal display device uses reflected light for display, the means to reduce reflection provided to the touch panel should not interrupt display image produced by the underlying reflective liquid crystal display device. For example, Japanese Laid-Open Patent Application No. 5-127822/1993 (Tokukaihei 5-127822) discloses that a touch panel, a quarter-wave plate, and a polarizer plate are stacked together to reduce reflection.
Among the aforementioned conventional techniques, the liquid crystal display device disclosed in Japanese Laid-Open Patent Application No. 55-48733/1980 is not suitable for a high resolution, high precision display, because despite the need to provide a quarter-wave plate between a liquid crystal layer and a reflector plate, it is difficult essentially to form a reflective film inside the liquid crystal cell.
In addition, the liquid crystal display device that operates in the homeotropic alignment mode disclosed in U.S. Pat. No. 4,701,028 has following problems. The homeotropic alignment, especially the inclined homeotropic alignment, is extremely difficult to control, and the control requires such a complex arrangement that is not suitable for mass production. Another shortcoming of the homeotropic alignment is its slow response.
In addition, coloring occurs with the aforementioned reflective-type parallel alignment mode due to small unevenness of the liquid crystal cell and the optical retardation compensator plate. The conventional arrangements, as discussed here, are likely to suffer from coloring in a dark state and failure to realize black and white display.
In addition, the arrangements disclosed in Japanese Laid-Open Patent Application No. 2-236523/1990 and Japanese Laid-Open Patent Application No. 4-116515/1992, although being capable of increasing the reflection in a bright state in comparison to the arrangement using two polarizer plates, still fail to realize a good black display due to great wavelength dependency of transmittance in a dark state.
In addition, ASID 97, although disclosing a display mode that enables a black and white display, does not disclose anything about the arrangement of the quarter-wave plate which is, described in this literature, to be fabricated for a broad band of spectrum.
In addition, according to a report made by Pancharatnam, three optical retardation compensator plates are required to obtain good circularly polarized light, which is not practical. In addition, detailed studies are yet to be made to combine this with liquid crystal display devices.
In contrast, the touch-panel-incorporating reflective liquid crystal display device, although its performance as a reflective liquid crystal display device has reached to a stage where it can be commercialized, still suffers from extremely poor visibility when used in a combination with a touch panel.
This is because, in the reflective display device, a single light source plays dual roles to cause reflection at the touch panel and to serve as a display light source for the display device, and decrease in visibility when used in a combination with a touch panel cannot be solved by removing the light that radiates from a light source (for example, a ceiling light) which cause reflection at the touch panel, or changing the direction of the light. This is a stark contrast to the transmissive liquid crystal display device and other light-emitting types of display devices with which this solution produces good results. A conclusion drawn from here is that the solution to the poor visibility is a key to a successful commercialization of the display device, as well as to that of a practical, low power consuming personal digital tool.
In addition, the arrangement of the touch panel disclosed in Japanese Laid-Open Patent Application No. 5-127822/1993 is effective in preventing reflection by means of the function of the quarter-wave plate; however, a typical quarter-wave plate is effective in preventing reflection only with respect to a particular wavelength in the visible range, and unavoidably less effective with respect to wavelengths that are immediately higher or lower than those particular wavelengths. Further, the brightness of a display is determined by a component of the polarized light that has travelled through the underlying display device, the component being in a transmission direction of a circular polarizer that is obtained as a combination of such a quarter-wave plate with a polarizer plate.
More specifically, when the underlying display device has substantially no polarization dependence (e.g., a white-Taylar type Guest-Host liquid crystal display device including dyestuffs added to its 360°-twisted liquid crystal), the reflection efficiency is, at maximum, half that of a display device having the same arrangement except that no touch panel is provided due to the transmittance of the polarizer plate placed on the front of the touch panel. Also, as another example, when the underlying display device utilizes linearly polarized light for a display (e.g., a TN or STN type liquid crystal display device including a polarizer plate further interposed in the space between the touch panel and the liquid crystal cell), the reflection efficiency is, at maximum, half that of a display device having the same arrangement except that no touch panel is provided. Further, in the last example, since the retardation caused by the quarter-wave plate depends on the wavelength of light, and the quarter-wave plate is sandwiched by polarizer plates, which causes tonal changes. In either case, brightness is insufficient, and is not suited for use in a combination with a reflective liquid crystal display device to which brightness improving means such as background light cannot be applied.
From what is laid above, it can be said that the touch panel described in Japanese Laid-Open Patent Application No. 5-127822 needs to be upgraded in its reflection preventing function. Additionally, the Laid-Open Patent Application does not disclose a suitable arrangement to utilize the daylight that has entered the touch panel for the reflective liquid crystal display device.